1. Field of the Invention
The present invention relates to an RF strip line resonator.
2. Description of the Related Art
RF strip line resonators are required in oscillatory circuits which are constructed using strip line technology and are required for specific applications. A significant field of application is, for example, radio-telecommunications technology in which radio-telecommunications are transmitted in the radio wave range. The subdivisions of radio-telecommunications technology which cover the radio wave range are, for example, radio technology, television technology, mobile radio technology and satellite technology.
In mobile radio technology, which is to be considered primarily below, there are a number of mobile radio systems for transmitting telecommunications, which systems differ in terms of
(a) the field of application (public mobile radio or non-public mobile radio)
(b) the transmission method (FDMA=Frequency Division Multiple Access; TDMA=Time Division Multiple Access; CDMA=Code Division Multiple Access;),
(c) the transmission range (from a few meters up to several kilometers),
(d) the frequency range used for the transmission, (800-900 MHz; 1800-1900 MHz).
Examples of this are the public GSM mobile radio system with a transmission range of several kilometers and a frequency range for telecommunications transmission between 800 and 900 MHz (Group Spxc3xa9ciale Mobile or Global Systems for Mobile Communications; cf. the publication entitled Informatik Spektrum [computing publication], Springer Verlag Berlin, Year 14, 1991, No. 3, pages 137 to 152, the publication by A. Mann: xe2x80x9cDer GSM-Standardxe2x80x94Grundlage fxc3xcr digitale europaische Mobilfunknetzexe2x80x9d [The GSM Standardxe2x80x94Basis for digital European mobile radio networks]) and the non-public DECT cordless system with a transmission range of several 100 meters and a frequency range for telecommunications transmission between 1880 and 1900 MHz (Digital European Cordless Telecommunication; cf. the publication entitled Nachrichtentechnik Elektronik [Telecommunications Electronics], Berlin, Year 42, No. 1, 1-2/1992, pages 23 to 29, and the publication by U.Pilger: xe2x80x9cStrukur des DECT-Standardsxe2x80x9d [Structure of the DECT standard]); both use the powerful TDMA transmission method.
The possibility of using RF strip line resonators in mobile radio systems is demonstrated below for the DECT cordless system. In the DECT cordless system which comprises, in the simplest case, a base station with at least one assigned mobile component, high frequency signals are required and processed in radio components with a transmitter/receiver structure.
FIG. 1 shows, for example, the known (publication: the publication entitled ntz, Vol. 46, Issue 10, 1993, pages 754 to 757xe2x80x94xe2x80x9cArchitekturen fxc3xcr ein DECT-Sende- und Empfangsteil: Ein Vergleichxe2x80x9d [Architectures for a DECT transmission and reception component: a comparison]) basic structure of a DECT radio component FKT according to the superheterodyne principle with double frequency conversion. Mixers MIS which mix a traffic signal (such as a transmission or reception signal) up or down (in other words, raise or lower the frequency of the traffic signal) by mixing with an oscillator signal are used for this frequency conversion. In order to generate the oscillator signal, oscillators OSZ1 and OSZ2, which have correspondingly constructed resonators for this, are usually used in the radio component FKT. In this context, the resonators used are preferably RF strip line resonators. A housing H is shown enclosing the component FKT.
FIG. 2 shows the known structure of an RF strip line resonator 1 which is constructed, for example, as a shortened quarter wave resonator. A quarter wave resonator 1 is arranged, for example, on a printed circuit board 2 with a substrate thickness ds (reference distance). The quarter wave resonator 1 has a strip line 10 which is directly connected at one endxe2x80x94by means of a through-plated hole DKxe2x80x94and is connected via a capacitor 3 at the other end, to a metallic conductor 11xe2x80x94in the present case a metallized conductor surfacexe2x80x94which is used here as an earth potential for the strip line 10. The strip line 10 and the metallic conductor 11 are arranged here on opposite faces of the printed circuit board 2. The strip line 10 has a length lST and a width bST by which, together with the capacitance of the capacitor 3, the method of forming the through-plated hole DK, the substrate thickness ds and the dielectric constant xcex5r of the printed circuit board 2, the resonant frequency of the quarter wave resonator 1 is determined. By means of the capacitor 3, the strip line resonator 1 is on the one hand adjusted in terms of the resonant frequency and on the other hand shortened in terms of the resonator length lST.
Owing to the dependence of the resonant frequency of the strip line resonator 1 on the parameters given above, the actual resonant frequency of the strip line resonator 1 is also determined by how precisely the strip line resonator 1 can be produced, i.e. how large the manufacturing tolerances are. Tolerances (xcex94ds) in the substrate thickness ds or quite generally in the distance between the strip line 10 and the metallic conductor 11 (difference between the reference distance ds and an actual distance dsxc2x1xcex94ds) prove particularly problematic.
Moreover, this problem is increased if the strip line resonator 1 described above is surrounded by a metallic housing or housing cover and it is also impossiblexe2x80x94for reasons of manufacturexe2x80x94for this metallic conductor to be arranged at a defined distance from the strip line.
An object on which the invention is based is to provide an RF strip line resonator in which changes in the resonant frequency of the resonator which occur owing to tolerances in the construction of the RF strip line resonator which are due to production and which influence the distance between the strip line and the metallic conductor are compensated.
This and other objects and advantages are achieved on the basis of the RF strip line resonator having a curved strip line which is arranged at a reference distance from a conductor characterized in that the strip line is curved and the curvature is dimensioned such that the displacement in the resonant frequency which is capacitively caused as a result of a deviation in distance between an actual distance and the reference distance is counteracted by an approximately equal inverse inductively caused displacement in the resonant frequency.
By virtue of the fact that a strip line of the RF strip line resonator is no longer of a stretched, as in the prior art, but is rather of a curved construction, eddy currents are induced in a metallic conductor which is located parallel to the strip line and is preferably constructed as a metallic surface. The eddy currents bring about a reduction in the inductance of the RF strip line resonator. The smaller the distance between the strip line and the metallic conductor becomes, the smaller this inductance becomes and similarly the larger the distance between the strip line and the metallic conductor, the larger the inductance. Since the shortening of the distance between the two conductors is however also accompanied by an increase in the capacitance of the RF strip line resonator and an increase of the distance between the two conductors is accompanied by a reduction in the capacitance of the resonator, with appropriate dimensioning of the curved strip line, the two aforesaid effects cancel one another out and the frequency of the RF strip line resonator is approximately stable with respect to the given fluctuations in distance.
Advantageous developments of the invention are provided by the strip line and the conductor being arranged on opposite sides of a printed circuit board. The printed circuit board is surrounded by an electrically conductive housing lid in one embodiment. The conductor is preferably constructed as a metallic surface which is used as ground potential for the strip line. The present RF strip line resonator is preferably used in a wireless telecommunications device. One use of the present RF strip line resonator is in a DECT cordless telephone. Another use is in a GSM mobile radiotelephone.